Your assumption on the letter sender is where your misunderstanding is occurring. In the Philosophers Stone, we find out Hagrid is in charge of sending Harry his letters. His first attempt added a letter addressed to Harry Potter blahblah, under the staircase, to the pile of muggle mail, whether sent through mugglemail or simply slipped into the mailbox/slot is another question.
So we have
A. letters were sent directly to Harry,
B. the amount of letters increased in an attempt to get one of them into Harry's hands past the Dursleys without direct intervention (Also Hagrid was allowed to use magic on this mission and he was most likely having fun (since he is technically not allowed to do magic, strictly speaking of course) ).
"I was allowed ter do a bit ter follow yeh an'get yer letters to yeh an' stuff -- one o' the reasons I was so keen ter take on the job --"
"I never expected this," he said, in a low, worried voice. " I had no idea, when Dumbledore told me there might be trouble gettin' hold of yeh, how much yeh didn't know. Ah Harry, I don' know if I'm the right person to tell yeh -- but someone's gotta -- yeh can't go off ter Hogwarts not knowin'."
C. Finally Hagrid does show up and personally deliver the final letter to Harry after the Dursleys made a run for it and were hiding on an abandoned island.
